This paper reviews the field of high-speed small-aperture modulators for applications in optical communications, with emphasis on electrooptic modulation. The capabilities and limitations of electrooptic modulators are discussed based on a review of the physical origin of the electrooptic effect. Thermal and photoconduction phenomena, which may severely limit the operation of practical devices, are emphasized. The modulation power and bandwidth limitations using various schemes of electrooptic interaction are derived and compared. It is shown that lumped modulators are capable of efficient modulation for bandwidths up to about 1 GHz for visible wavelengths and are also attractive for their simplicity. For broader bandwidth capability the traveling wave or zigzag types of interaction become more efficient but with added complexity. Finally, acoustooptic and magnatooptic modulators are briefly discussed and compared with electrooptic modulators.

Modulators for optical communications

We describe methods for time-resolved imaging of the complex coherence of a spectral scene at one or more optical delays using electro-optically modulated polarization interferometers. By encoding the coherence information at harmonics of the electro-optic modulation frequency, it can be possible to obtain unambiguous spatio-temporal information about important physical parameters that govern the spectral content of the scene. We discuss the physical principles upon which the instrument is based and describe some applications in plasma spectroscopy, including Doppler tomography, Zeeman spectroscopy and relative line intensity measurements.

/pdf/time-resolved-two-dimensional-plasma-spectroscopy-using-1pe0jprn74.pdf

Time-resolved two-dimensional plasma spectroscopy using coherence-imaging techniques

The method of polarization-rotated reflection is applied to sensors with polarization-maintaining optical fibers. By the use of this scheme, stable measurement can be realized because the fluctuation of the light propagation characteristics in the fiber is canceled automatically without any additional active phase compensation techniques. Several kinds of sensors were designed for the measurement of different physical values. A magnetic field sensor and a temperature sensor were built and the performances were tested to confirm their basic features experimentally.

Optical fiber sensors using the method of polarization-rotated reflection

Ferroelectric materials have found extensive application in optoelectronic devices for uses in optical communication, memory, display and coherent optical processing. Specific device applications include modulators, beam deflectors, light valves and holographic storage media. These device applications are reviewed with emphasis on progress made since June 1971, and on the competition provided by such technologies as acoustooptics, magnetooptics and liquid crystals.

Optical applications of ferroelectrics

This paper aims at two practical design issues encountered with electro-optic amplitude modulators, namely (1) the long-term optical power drift when a single electro-optic crystal is used and (2) the mechanical resonances of the crystal that limit the control bandwidth. We investigated an active control scheme using a single crystal to offset the drifts of different origins such as polarization, crystal birefringence, and electronics. The control system worked continuously for at least 15 h without interruption and a power instability of 8 x 10(-4) was achieved in a 3-h measurement. This technique can be employed in electro-optic applications where two matched crystals are usually used to compensate the birefringence-related drifts. Meanwhile, mechanical resonances in two crystals (LiNbO3 and KTiOPO4) were mapped out and their influences on the control bandwidth were quantitatively investigated. Residual noises of two intensity-stabilized lasers (a Helium-Neon laser and a dye laser) were analyzed in detail. (c) 2012 Elsevier Ltd. All rights reserved.

Long-term and wideband laser intensity stabilization with an electro-optic amplitude modulator

LiTaO3 optical modulators have been developed using the technique of self-compensation for therma stability. By using two crystals in cascade, with optic axes 180° apart, and a half-wave plate between them with axis at 45°, the birefringence of each crystal is equal but opposite and there is no net change in birefringence with temperature. Aside from thermal stability, the optical modulator described damps out mechanical resonances caused by the piezoelectric properties of the crystals, and its thin-film substrate mounting is applicable to wideband modulation. Experimental measurements show that 95% modulation depth can be attained over a 25° C temperature range with virtually no change in the amplitude or phase of the modulated signal.

Fabrication of a lithium tantalate temperature-stabilized optical modulator.

Light pulses from a mode-locked He–Ne laser have been modulated by a LiTaO3 electrooptic crystal mounted on a thin film substrate. The crystal was driven by pulses from a GaAs Gunn effect diode. Amplitude modulation of 20% has been achieved at 2 GHz for a single pass through the modulator.

Gigahertz optical modulation.

LiTaO3 optical modulators have been developed using the technique of self-compensation for therma stability. By using two crystals in cascade, with optic axes 180° apart, and a half-wave plate between them with axis at 45°, the birefringence of each crystal is equal but opposite and there is no net change in birefringence with temperature. Aside from thermal stability, the optical modulator described damps out mechanical resonances caused by the piezoelectric properties of the crystals, and its thin-film substrate mounting is applicable to wideband modulation. Experimental measurements show that 95% modulation depth can be attained over a 25° temperature range with virtually no change in the amplitude or phase of the modulated signal.

M. R. Biazzo

Papers

Fabrication of a lithium tantalate temperature-stabilized optical modulator.

Gigahertz optical modulation.

Fabrication of a Lithium Tantalate Temperature-Stabilized Optical Modulator